Method for processing a measurement signal from a pressure measurement cell, and a measurement cell arrangement

ABSTRACT

Method for determining a pressure in a pressure cell, wherein the method consists in the fact that a measuring signal (x) is determined, which is at least proportional to a measured pressure in the pressure cell, that an output signal (y) is produced from the measuring signal (x) using a filter unit (10) having a transfer function, in that a noise signal contained in the measuring signal is at least reduced, and preferably eliminated, that a time change of the measuring signal (x) is determined, and that the transfer function is set in a function of the time change of the measuring signal (x). In addition, a measuring cell arrangement is provided.

The present invention relates to a method for processing a measuringsignal from a pressure measuring cell and a measuring cell arrangementwith a pressure measuring cell.

It is known to measure pressures or pressure differences in such a waythat a thin membrane is subjected to pressure and the deflectionresulting therefrom is measured. A known and suitable method to measurethe deflection of such membranes consists of forming the membranearrangement as a variable electrical capacitance, wherein the change inthe capacitance, which correlates with the change in pressure, isevaluated via an electronic measuring system. The capacitance is formedin that the thin, flexible membrane surface is arranged at a smalldistance from a further surface of a body and both mutually oppositesurfaces are formed to be electrically conductive. If the membrane andthe body consist of non-conductive dielectric material, the surfaces arecoated with an electrical coating for example, so that capacitorelectrodes are formed. The membrane and/or the body can also be formedthemselves of electrically conductive material, wherein in this case thesurfaces again form the capacitor electrodes. If pressure is applied tothe membrane, the distance between the two electrodes changes as aresult of deflection, which leads to a change in capacitance that can beevaluated. Sensors of this kind are produced in large numbers fromsilicon, for example. Both the flat base body and the membrane oftenconsist entirely of silicon. There are also other embodiments withcombined composition of materials, e.g. silicon with a glass base. Thesensors can thus be produced at low cost. Pressure sensors of this typecan for the most part only be used for higher pressure ranges in therange of approximately 10⁻¹ mbar to a few bars. A high resolution atlower pressures from approximately 10⁻¹ mbar can no longer be realizedwith silicon material. Sensors of this kind are not suitable for typicalvacuum applications. For the various vacuum processes to be monitored,pressure measurements are often carried out in a vacuum betweenatmospheric pressure and 10⁻⁶ mbar. Such measurements require highsensitivity with high resolution and good reproducibility of the vacuumpressure measurement, which can only be provided by specially designedmeasuring cells which completely deviate from the design of thehigh-pressure measuring cell.

Capacitive membrane pressure measuring cells which are made ofcorrosion-proof materials such as Al₂O₃ are especially suitable forvacuum pressure measurement. A known capacitive vacuum measuring cell,which is substantially completely made of ceramics and is largelycorrosion-proof, is described in EP 1 070 239 B1. In order to enable themeasurement of very low pressures to 10⁻⁶ mbar with high precision, avery thin ceramic membrane with a thickness of 60 μm is used, forexample, which is arranged in a tension-free and symmetric manner in aceramic housing.

The distance of the capacitor electrodes or the membrane surfaces fromthe surface of the housing body preferably lies in the range of 2 to 50μm. The diameters of such membrane pressure measuring cells preferablylie in the range of 5 to 80 mm. The thus formed capacitances to bemeasured lie in the range of 10 pF to 32 pF. Thanks to new electronics,we can now measure capacitances in a range of 5 pF to 1000 pF. Themeasured capacitance is used as a measure for the pressure to bemeasured. This capacitance changes accordingly under apressure-dependent deflection of the membrane, by means of which thepressure applied to the membrane can be detected. This measurement ofthe capacitance must occur in a very precise way and is not easy tocarry out in the case of very low capacitance values because the lowcapacitances lead to the consequence that the changes in capacitancecaused by the changes in the pressure are extremely small. As a result,the electrical signals generated or derived therefrom are exceptionallylow and thus susceptible to disturbances.

Accordingly, high demands are thus placed on the signal processingsystems for processing pressure signals according to the explanationsmade above. Furthermore, filter algorithms are used for optimizing theproperties of the measured pressure signals for further use, e.g. forcontrolling the pressure in process chambers. This is an attempt toprovide a filter algorithm which simultaneously achieves twocontradictory objectives for processing pressure signals. Firstly, atransient response, for example after a step-like change in themeasuring signal, should be completed as rapidly as possible, i.e., theoutput signal of the filter should lead as quickly as possible to astable output signal. As a result, any necessary action due to a changein pressure can be initiated as rapidly as possible. Secondly, apotential noise signal must be suppressed as strongly as possible by thefilter algorithm. Thus, this requires a filter that is as fast aspossible according to the first condition, whereas according to thesecond condition a rather slow filter is desirable.

Numerous efforts are known to provide a filter algorithm and thus atransfer function for a filter for processing the measuring signal so asto achieve the two contradictory objectives. The known filter algorithmsare based on compromises, which in the present application of pressuremeasurement using highly sensitive sensors do not lead to satisfactoryresults.

U.S. Pat. No. 5,838,599 describes a variant for a filter, which permitsboth short transient responses during a rapid change of the input signaland a good reduction in the noise signal components in the input signalin the steady state.

Furthermore, reference is made to US 2013/0016888 A1, which discloses acomplex computational method with a linear filter for eliminating noise.

It is the object of the present invention to provide a simple method forprocessing a measuring signal in which a distinct suppression of thenoise signal is achieved, but simultaneously allows a rapid reaction torelevantly changing measuring signals.

This object is achieved by the features of claim 1. Advantageousembodiments as well as a measuring cell arrangement with a pressuremeasuring cell are provided in the further claims.

The method according to the invention for determining a pressure in apressure cell lies in the fact

-   -   that a measuring signal is determined that is at least        proportional to a measured pressure in the pressure cell,    -   that an output signal is produced from the measuring signal        using a filter unit having a transfer function, in that a noise        signal in the measuring signal is at least reduced and        preferably eliminated,    -   that a time change in the measuring signal is determined, and    -   that the transfer function is set in the time change function of        the measuring signal.

One embodiment of the method according to the invention lies in the factthat the pressure in the pressure cell is set at least proportional tothe output signal. A closed control system is thereby obtained that isextremely stable and robust.

The output signal is linear and thus excellently suited as the actualvalue for modern controllers (state controller).

One embodiment of the method according to the invention lies in the factthat a transfer function has at least in a first order a low passcharacteristic, wherein its time constant is set in a function of thetime change of the measuring signal.

Further embodiments of the method according to the invention consist inthe fact that an average value of the measuring signal is determined,that a difference signal is determined by a difference formation betweenthe measuring signal and the average value of the measuring signal, andthat the time change of the measuring signal is derived at least fromthe difference signal.

Further embodiments of the method according to the invention consist inthe fact that the average value of the multiple substrate is determinedusing an exponential average filter, which for a time-discrete measuringsignal is defined by

f _(n)=β₁ ·x _(n)+(1−β₁)·f _(n−1)

wherein f is the time-discrete output signal, β₁ is a variable, x thetime-discrete measuring signal, and n a time-dependent index, whereinthe variable β₁ preferably has a value between 1 and 0, especiallypreferably between 1 and 0.1, and most especially preferably between0.85 and 0.95.

Further embodiments of the method according to the invention consist inthe fact that the time change of the measuring signal is determined byforming an average value of the difference signal.

Further embodiments of the method according to the invention consist inthe fact that the time change of the measuring signal is determinedusing an exponential average filter, which for a time-discretedifference signal is determined by

(Δx/ΔT)_(n)=β₂ e _(n)+(1−β₂)·(Δx/ΔT)_(n−1)

wherein (Δx/ΔT)_(n) is the time-discrete change in the measuring signal,β₂ is a variable, e is the time-discrete difference signal, and n atime-dependent index, wherein the variable β₂ preferably has a valuebetween 1 and 0, especially preferably between 0.5 and 0.01, and mostespecially preferably between 0.5 and 0.15.

Further embodiments of the method according to the invention consist inthe fact that the time constant of the transfer function of atime-discrete system is determined by

$\tau = {\Delta \; {T \cdot \frac{1 - \alpha}{\alpha}}}$

wherein ΔT corresponds to the sampling interval in the time-discretesystem and α is a variable those value is at least proportional to thetime change of the measuring signal, but does not go below a minimalvalue of α_(min), and does not exceed a maximal value of α_(max),wherein the minimal valve α_(min)preferably is between 0.0 and 0.1,especially preferably between 0.0 and 0.01, while the maximal valuea_(max) lies most preferably between 0.3 and 1.0.

Further embodiments of the method according to the invention lie in thefact that the transfer function is defined by the formula

y _(n) =α·x _(n)+(1−α)·y _(n−1)

wherein y is the time-discrete output signal, x is the time-discretemeasuring signal, α is a variable whose value depends on the time changeof the measuring signal, and n is a time-dependent index.

Still other embodiments of the method according to the invention consistin the fact that the measuring signal is processed in a fast path forproducing an output pulse, wherein the output pulse of the fast path isactive at least as long as the measuring signal change measured in atmost three sampling intervals is greater than the noise measured in thesame period in the measuring signal or in the measuring signal change.

Still further embodiments of the method according to the invention linein the fact that the measuring signal further is processed in a slowpath for producing a switching signal, wherein the switching signal ofthe slow path is active at least as long as the change of the measuringsignal measured for longer than 2*TS is greater than the noise in themeasuring signal measured in the same time period, or is in themeasuring signal change, wherein TS is a predetermined pulse width ofthe output pulse, and that the variable α receives a value depending onan OR operation between the output pulse and the switching signal.

Still further embodiments of the method according to the inventionconsist in the fact that the variable α assumes at least after apredetermined transition time after a switching process either the valueα₁ or the value α₂, wherein the value for α₁ preferably lies in theregion of 0.01 to 0.9 and wherein the value for α₂ preferably lies inthe region of 0.0001 to 0.01.

Still further embodiments of the method according to the inventionconsist in the fact that switching from a value α₁ to a value α₂ occursover a timespan F_(in) and/or that switching from a value α₂ to a valueα₁ occurs over a timespan F_(out).

Furthermore, the invention relates to a measuring cell arrangement witha pressure cell and a membrane pressure cell functionally connected tothe pressure cell, producing a pressure-dependent measuring signal thatis applied to a filter unit having a transfer function for producing anoutput signal, wherein a time change of the measuring signal may bedetermined, and the transfer function in the function of the time changeof the measuring signal is adjustable.

An embodiment of the measuring arrangement according to the inventionlies in the fact that the output signal may be used for setting thepressure in the pressure cell, and here in particular for setting thepressure in a process chamber.

An embodiment of the measuring arrangement according to the invention,lies in the fact that the transfer function, at least in a first order,comprises a low pass characteristic, wherein its time constant in thefunction of the time change of the measuring signal is adjustable.

Further embodiments of the measuring arrangement consist in the factthat an average value of the measuring signal may be determined, that adifference signal may be determined through a difference formationbetween the measuring signal and the average value of the measuringsignal, and that the time change of the measuring signal may be derivedat least from the difference signal.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the average value of the measuringsignal may be determined using an exponential average filter, which isdefined for a time-discrete measuring signal by

f _(n)=β₁ ·x _(n)+(1−β₁)·f _(n−1)

wherein, f is the time-discrete average value of the measuring signal,β₁ is a variable, x is the time-discrete measuring signal, and n is atime-dependent index, where the variable β₁ preferably has a valuebetween 1 and 0, especially preferably between 1 and 0.1, and mostespecially preferably between 0.85 and 0.95.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the time change of the measuringsignal may be determined by forming an average value of the differencesignal.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the time change of the measuringsignal may be determined using an exponential average filter, which isdefined for a time-discrete difference signal by

(Δx/ΔT)_(n)=β₂ e _(n)+(1−β₂)·(Δx/ΔT)_(n−1)

wherein (Δx/ΔT)_(n) is the time-discrete time change of the measuringsignal, β₂ is a variable, e_(n) is the time-discrete difference signaland n is a time-dependent index, wherein the variable β₂ preferably hasa value between 1 and 0, especially preferably between 0.5 and 0.01, andmost especially preferably between 0.05 and 0.15.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the time constant of the transferfunction of a time-discrete system is defined by

$\tau = {\Delta \; {T \cdot \frac{1 - \alpha}{\alpha}}}$

wherein ΔT corresponds to the sampling interval in the time-discretesystem and α is a variable whose value is at least proportional to thetime change of the measuring signal, but is not less than a minimalvalue α_(min) and is not greater than a maximal value α_(max), whereinthe minimal value α_(min) preferably lies between 0.0 and 0.1,especially preferably between 0.0 and 0.01, while the maximal valuea_(max) most preferably lies between 0.3 and 1.0.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the transfer function is defined bythe formula:

y _(n) =α·x _(n)+(1−α)·y _(n−1)

wherein y is the time-discrete output signal, x is the time-discretemeasuring signal, α is a variable whose value depends on the time changeof the measuring signal, and necessary is a time-discrete index.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the measuring signal is applied to afast path for producing an output pulse, wherein the output pulse of thefast path is active at least as long as the measuring signal changemeasured during at least three sampling intervals is greater than thenoise in the measuring signal or in the measuring signal change measuredin the same period of time.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the measuring signal is furtherapplied to a slow path for producing a switching signal, wherein theswitching signal of the slow path is active at least as long as thechange in the measuring signal measured for longer than 2*TS is greaterthan the noise in the measuring signal or in the measuring signal changemeasured in the same time period, wherein TS is a minimal pulse width ofthe output signal and that the variable α acquires a value depending onan OR operation between the output pulse and the switching signal.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that the variable α after a predeterminedtransition time following a switching process assumes either the valueα₁ or the value α₂, wherein the value for α₁ preferably lies in therange of 0.01 to 0.9, and wherein the value for α₂ preferably lies inthe range of 0.0001 to 0.01.

Further embodiments of the measuring arrangement according to theinvention consist in the fact that a transition unit is provided betweenthe filter unit and the decision unit, in which switching from a valueof α₁ to a value of α₂ occurs over a timespan F_(in) and/or switchingfrom a value of α₂ to a value α₁ over a timespan of F_(out).

It is pointed out that the above embodiments may be combined in anymanner desired. Only those combinations of embodiments are excluded thatwould by combination lead to a contradiction.

Below embodiments of the present invention are explained in detail bydrawings. Wherein:

FIG. 1 shows a measuring cell arrangement with a membrane pressure cell,connected to a process chamber, with which a measuring signal isdetermined, which after processing according to the invention issupplied to a valve,

FIG. 2 shows a block diagram of the signal processing unite according toFIG. 1 with a calculation unit for processing of the measuring signal,

FIG. 3 shows a block diagram of a first embodiment of the calculationunit according to FIG. 2,

FIG. 4 shows a block diagram of a second embodiment of the calculationunit according to FIG. 2,

FIG. 5 shows a signal flow diagram that depicts the method stepsoccurring in a decision unit,

FIG. 6 shows a block diagram of a further embodiment for the calculationand decision unit according to the invention with a fast and a slowsignal path,

FIG. 7 shows a block diagram for the fast signal path according to theinvention according to FIG. 6,

FIG. 8 shows a block diagram for the slow signal path according to theinvention according to FIG. 6, and

FIG. 9 proceeding from FIG. 2, a further embodiment with a transitionunit between the calculation unit and filter unit.

FIG. 1 shows in a highly simplified block diagram a measuring cellarrangement with a process chamber 1, a membrane pressure cell 2, avacuum pump 3, a signal processing unit 4, a control unit 5, a valveactuator 6, and a valve 7. The membrane pressure cell 2 is used fordetermining the pressure in the process chamber 1, in which a pressurepredetermined in accordance with the specification of a vacuum processis set. Vacuum processes comprise the most diverse methods, such as forexample coating procedures, etching procedures, heat treatment ofworkpieces, etc. Vacuum processes are also often carried out withsupporting gases, which are needed both actively as a reactive gas or asan inert case in the process. The gases are supplied for this purpose tothe process chamber 1 via the valve 7 controlled by the valve actuator6, with which the gas supply and the pressure in the process chamber 1may be controlled. By means of the membrane pressure cell 2, a measuringsignal x is produced which is processed in the signal processing unit 4and the control unit 5 to a control signal s for the valve actuator 6.For process control, it is necessary that the membrane pressuremeasuring cell 2 on the one hand be as precise as possible, but on theother hand also makes quick measurements in order to be able to react asquickly and precisely as possible to pressure changes in the processchamber 1.

It is also conceivable—in a simplified embodiment of the presentinvention—that the output signal y of the signal processing unit 4 isnot used for controlling the pressure in a process chamber. In this casethis is not a closed system but an open system. Here a pressure ismeasured in a pressure cell of any desired type—similarly to in theprocess chamber according to FIG. 1—with a pressure cell 2. Themeasuring signal x measured with the pressure cell 2 is likewiseprocessed in a signal processing unit 4 in order to obtain an outputsignal y that is stable and noise-free but reacts quickly to changes.

The invention now relates—again with consideration of the embodimentsaccording to FIG. 1—two processing of the measuring signal x in thecontext of conditions present in a vacuum process and above all isintended for optimal signal processing of the measuring signal x, as itcan occur as the pressure signal in such vacuum processes. Here thesignal processing in the signal processing units 4 can basically be inanalog or digital form, wherein in the following we shall not go intothe special preparations that are carried out when signal processing isin analog or digital form as such preparations (analog/digitalconversion, filtering to avoid aliasing, selection of sampling frequencyetc.) are sufficiently known to the person skilled in the art.

The output signal y of the signal processing unit 4 is processed furtherin the control unit 5 for example with a so-called P-, PI-, PID- orstate controller. The controller implemented in the control unit 5 isresponsible in particular for optimal tracking of the control signal sfor the valve actuator 6 or for the valve 7.

Basically, the statements regarding processes in the signal processingunit 4 and their block diagram images both for the embodiments in aclosed system and for the embodiments in an open system are valid.

FIG. 2 shows schematically and in simplified form a block diagram forillustration of the processing steps that are carried out in the signalprocessing unit 4 (FIG. 1) according to the present invention. Forimplementing the processing steps of the algorithm according to theinvention for example a signal processor is used, which is accordinglyprogrammed. Naturally the signal processor can also perform other tasksif the processor capacity is sufficient. In particular it is conceivablethat the controller of the control unit 5 is implemented in the samesignal processor.

As is plain from FIG. 2, the measuring signal x of a filter unit 10 issupplied that produces the output signal y. The filter unit 10 with themeasuring signal x and the output signal y form the actual signal pathof the signal processing unit 4 (FIG. 1). The remaining components thatare still to be explained such as the calculation unit 11 and thedecision unit 12 are provided for establishing the characteristic of thefilter unit 10.

The filter unit 10 has a filter characteristic that is defined in atime-discrete system for example according to the following equation:

y _(n) =α·x _(n)+(1−α)·y _(n−1)

Here y is the time-discrete output signal, x is the time-discretemeasuring signal, n is a time-dependent index, and α is a variable whosevalue decisively determines the time constant of the filter unit 10. Theobject of the present invention is optimal setting of the value for thevariable α in such a way that a noise signal in measuring signal x issuppressed to the extent possible or even eliminated, but at the sametime a changing pressure in the process chamber is identified so as tobe able to react to it with the appropriate speed.

The mentioned equation with the variable α has as the filtercharacteristic for suppression of the noise signal portion a low passcharacteristic, wherein the time constant τ can be defined for afirst-order filter as follows:

$\tau = {\Delta \; {T \cdot \frac{1 - \alpha}{\alpha}}}$

The choice of values for the variable α is decisive for the presentinvention. When there is a stable pressure value, if the measuringsignal x receives only a noise signal, the value of α is to be chosen assmall as possible (α_(min)), for example 0.01. In this way, the noisesignal presence in the measuring signal x is maximally suppressed andthe filtered output signal y is preferably suited for use in thedownstream controller of the control unit 5 (FIG. 1), for a stableoutput signal leads to lower activity of the valve actuator 6 or of thevalve 7 and thus to a reduced load on these components were by theirprobability of failure with respect to known systems is considerablyreduced.

On the other hand, a change in the measuring signal x based on an actualpressure change in the process chamber must be detected without delay,which makes a different value for the variable α necessary, namely forexample of value for α between 0.3 and 1.0 (α_(max)).

The adjustment of the value for the variable α is implemented accordingto the invention depending on the time change of the measuring signal x,which is explained below in detail.

FIG. 3 shows in a schematic illustration a block diagram of a firstembodiment for defining the time change of the measuring signal x.Essentially the measuring signal x is smoothed out in a smoothing unit13, for example using an average filter. It has been shown that here inparticular a so-called exponential average filter is especially suited.The output signal f of this filter is sent to a difference unit 14,which determines the difference from the unchanged measuring signal x,which is below also defined below as a difference signal e.

The difference signal e is a measure for the time change of themeasuring signal x and is used in this embodiment according to theinvention for setting the value for the variable α in the filter unit 10(FIG. 2), wherein at most scaling is still required.

The smoothing unit 13 implemented by means of an exponential averagefilter is defined by the recursive formula

f _(n)=β₁ ·x _(n)+(1−β₁)·f _(n−1)

wherein f is the time-discrete output signal, β₁ is a variable, x is thetime-discrete measuring signal and n is a time-dependent index, whereinthe variable β₁ with the exponential average filter for producing thedifference signal f preferably has a value between 1 and 0, especiallypreferably between 1 and 0.1, and most especially preferably between0.85 and 0.95.

The calculation unit 11 according to FIG. 2 in the embodiment accordingto FIG. 3 thus produces a difference signal e as follows:

e _(n) =f _(n) −x _(n)

FIG. 4 shows, again in schematic representation, a block diagram of afurther embodiment of the calculation unit 11 (FIG. 2). This is aboutproduct of the time change of the measuring signal x, which may also beterms Δx/ΔT, in two stages, wherein the first stage is identical to theindividual stage according to FIG. 3 According to FIG. 4, a furtherprocessing of the difference signal e with an average filter 14 iscarried out, which average filter 15 can again for example beimplemented as an exponential average filter. Consequently, the samedefinitions apply; see above formulations, as they were as early as thefirst stage. Individually the value for the variable β, which wasaccordingly designated β2, has a different value: the variable β₂preferably acquires with the exponential average filter for determiningthe time change of the measuring signal x a value between 1 and 0,especially preferably between 0.5 and 0.1, most preferably a valuebetween 0.5 and 0.15.

The time change Δx/ΔT of the measuring signal x can thus be defined asfollows from the time-discrete difference signal e:

(Δx/ΔT)_(n)=β₂ e _(n)+(1−β₂)·(Δx/ΔT)_(n−1)

wherein (Δx/ΔT) is the time-discrete time change of the measuring signalx, e_(n) is the time-discrete difference signal e, and n istime-dependent index.

Also in this embodiment of the present invention, the time change Δx/ΔTor (Δx/ΔT)_(n) hereby obtained of the measuring signal x at most has tobe scaled, as was already explained in connection with the embodimentaccording to FIG. 3.

FIG. 5 shows a further embodiment of the present invention, wherein thisis a special embodiment of the decision unit 12 (FIG. 2). According tothe embodiment already explained with reference to FIGS. 3 and 4, thedecision unit 12 contains at most a scaling of the time change Δx/ΔT ofthe measuring signal x obtained by the calculation unit 11. Incontinuation of this embodiment of the invention, it is recommended thatthe range of values for the values a be limited to the lower and upperend. Accordingly, in accordance with the further embodiments of thepresent invention shown in FIG. 5, it is provided that an α_(min) and anα_(max) be provided, wherein these take effect in accordance with theflow diagram shown in FIG. 5:

The time change Δx/ΔT of the measuring signal x is scaled with a factork (as already explained in connection with the embodiments shown inFIGS. 3 and 4). The scaled time change Δx/ΔT is designated as α′. Therenow follows a series of decisions that have the objective of limitingthe establishment of α within a range of values between a minimal valueα_(min) and a maximal value α_(max). Between the extreme values α_(min)and α_(max), the value of a is accordingly set to the result of thecalculation unit 11 (FIG. 2), whether the latter was implementedaccording to FIG. 3 or according to FIG. 4.

FIG. 6 shows a further embodiment of the present invention, wherein herein schematic representation a block diagram is shown for the calculationunit 11 and the decision unit 12 shown in FIG. 2, which in FIG. 2 areenclosed in a broken line, and below may also be designated as the slopedetermination unit 50.

All of the following specific numerical examples (in particular for thetime constant τ) procedure from a typical sampling time ΔT (which mayalso be termed the cycle time) of 1 ms. Naturally the sampling time ΔTof 1 ms only serves as an example. The sampling time ΔT is basicallyselected within the scope of the available calculating performance andthe necessary reaction time of the entire system.

As is plain in FIG. 65, the slope determination unit 50 initiallycomprises two functional blocks—namely the “fast” block 20 and the“slow” block 30. These two functional blocks 20 and 30, which are yet tobe explained, are also designated the fast path 20 and the slow path 30.

In terms of quality, the following principles can be formulated withregard to parameters α₁ and α₂ of filter unit 10 (FIG. 2):

Measuring signals x, which do not change in the time function (i.e., ifno pressure changes are present), can be filtered intensely so as tomaintain a maximal noise suppression. It has been shown that for theparameter α₂, values between 0.0001 (τ˜10 s) and 0.01 (τ˜100 ms) aresuited. A preferred value for the parameter α₂ here is 0.001 (τ˜1 s).

Measuring signals x that change in a function of time (i.e., if pressurechanges are present) need to be less intensely filtered. In this case,the parameter α₁ defines the damping factor. This is therefore typicallychosen to be larger than the parameter α₂. It has been shown that forthe parameter α₁, values between 0.1 (τ˜100 ms) and 0.9 (τ˜0.1 ms) aresuitable. A preferred value for the parameter α₁ here is 0.1 (τ˜9 ms).

As already mentioned, the slope determination unit 50 initially consistsof the two functional blocks “fast” 20 and “slow” (30, wherein thefunctional block “fast” produces an output signal FC for fast changesand the functional block “slow” 30 an output signal SC for slow changes,from which a control signal SW is obtained by an “OR” operation asfollows:

SW=FC OR SC

The result of an active control signal SW—as follows from the flowdiagram of FIG. 6 after the OR gate is that the value α₁ is used in thefilter unit 10 (FIG. 2). On the other hand, α₂ is used in the filterunit 10 when the control signal SW is inactive.

The functional block “fast” 20 detects and reacts within a samplinginterval ΔT (wherein the sampling interval ΔT again is 1 ms, forexample) to fast changes of the measuring signal x, but is relativelyinsensitive to slow or constant measuring signal changed. The slow orconstant measuring signal changes are detected by the functional block“slow” 30.

The boundary between slow and fast measuring signals x is indicated bythe functional block “fast” 20:

If the frequency of the measuring signal x is smaller than

$\frac{1}{2 \cdot {TS}},$

from the standpoint of the functional block “fast” 20 this is a slowmeasuring signal x; otherwise it is a fast measuring signal x. Themeaning of these statements and the resultant reaction will be examinedin detail in connection with the explanations of the functional blocks20 and 30.

FIG. 7 shows the functional block “fast” 20 in accordance with FIG. 6.The measuring signal x is sent to the calculation unit 11, which isshown in FIG. 4 and is explained in detail. Accordingly, thosestatements are also valid for this embodiment of the invention.

The output signal Δx/ΔT of the calculation unit 11 is sent to a valueunit 21, in which the value of Δx/ΔT is determined and sent to anaddition unit 25. In a further value unit 22, the value of thedifference signal e likewise determined in the calculation unit 11 isobtained. The value signal |e| is then again smoothed in an averagefilter 23 with the parameter β₃ in accordance with the followingformula:

h _(n) =β ₃ ·|e _(n)|+(1−β₃)·h _(n−1)

wherein the output signal h_(n) after scaling with the factor CF in amultiplication unit 24 is sent to the additional unit 25, where thedifference between the value signal

$\left\lceil \frac{\Delta \; x}{\Delta \; T} \right\rceil$

and CF·h is determined. The result is sent to a threshold value detector26, which produces a trigger when a predetermined threshold value isexceeded, which is sent to a monoflop 27. The monoflop 27, which isformed for example as a retriggerable monoflop, after receipt of atrigger at the input, produces an output pulse CF, whose length may beset over the pulse width TS. In this regard, “retriggerable” means thata trigger arriving during the time process restarts the internal time ofthe monoflop each time, and the active switching state is accordinglyextended in time.

As already explained above, the signal Δx/ΔT constitutes a measure forthe change in the measuring signal x. By filtering of the amount of thedifference signal e with the average filter 23 and subsequent scalingwith the factor CF, the signal CF·h is obtained. This is now a measurefor the “basic noise” of the measurement of the measuring signal changeΔx/ΔT. By comparing the signals CF·h and the amount of Δx/ΔT one thusobtains the binary control signal “trigger,” which is used to controlthe monoflop 27.

It has been shown that the damping factors β₁ and β₂ and β₃ inparticular should have the following values:

For β₁ in a range of 0.1 to 0.001 (τ˜9 ms to 1 s), especially 0.01(τ˜100 ms) as a typical value; for β₂ in the range of 01 to 0.001 (τ˜9ms−1 s), especially 0.01 (τ˜100 ms) as a typical value, and for β₃ inthe range of 0.01 to 0.0001 (τ˜100 ms to 10 ms), especially 0001 (τ˜1 s)as a typical value.

Proceeding from the pressure monitoring and pressure setting systemshown in FIG. 1, the following can be established: each fast pressurechange (pressure jump) in the process chamber 1 produces a “triggerpulse.” The width of this pulse depends on the selected damping factorsβ₁ and β₂. Especially small pressure changes in real vacuum systems havetime constants in the range >10 ms. In order to ensure that there is noswitching within a pressure change (flank) between the fast and slowfilter, a retriggerable monoflop 27 is used to ensure that the outputpulse FC has at least the pulse width TS. The preferred value range forthe pulse width TS lies for example between 50 ms (especially withsmall, fast vacuum systems) and 5 s (especially for a large, slow vacuumsystems). A typical value for the pulse width TS is 500 ms. The scalingfactor CF for example has a value of 0.15.

For measuring signals x which have frequencies smaller than

$\frac{1}{2 \cdot {TS}},$

the functional block “fast” 20 does not respond or responds with lowerreliability than desirable. This is in particular because the pulsewidth TS of the monoflop 27 (FIG. 7) is too short so as to be able tocover a full signal period. Measuring signals x with a frequency ofsmaller than

$\frac{1}{2 \cdot {TS}}$

are therefore in the embodiment shown in FIG. 6 processed “slowly” bythe functional block. It is expressly pointed out, however, that alreadywith the functional block “fast” 20 alone—i.e., without the functionalblock “slow” 30—very good results can be obtained.

One embodiment for the functional block “slow” 30 (see FIG. 6) is shownin FIG. 8. Here the functional block “slow” 30 is set up for measuringsignals x with a frequency of smaller than

$\frac{1}{2 \cdot {TS}},$

wherein the frequencies typically are small than 1 Hz, assuming a pulsewidth TS of 500 ms for example. The functional block “slow” 30calculates a switching signal SC as an outpatient as follows:

${SC} = \left\{ \begin{matrix}{{{active}\mspace{14mu} {if}\mspace{14mu} {SSN}} < {SS}} \\{{{inactive}\mspace{14mu} {if}\mspace{14mu} {SSN}} \geq {SS}}\end{matrix} \right.$

wherein SS is a measure for the change of the measuring signal x over alonger time period, which for example is longer than 2·TS (i.e., doublethe pulse width TS) and thus typically amounts to seconds, and whereinSSN is a measure for the noise of the measuring signal x. Both SSN andSS are determined using the average filters of the already describedtype. The transfer function of the average filter was explained inconnection with the description of the filter unit 10 of FIG. 2.

SS is obtained with the further average filters 35 and 38 (FIG. 8)analogously to the average filters 13 of FIGS. 4 and 15 of FIG. 7. Theonly difference lies in the size of the damping factors β₅ and β₆, whichare no optimized for smaller frequency ranges.

It has been shown that the damping factors β₅ and β₆ should especiallyhave the following values:

For β₅ in the range of 0.01 to 0.0001 (τ˜100 ms to 10 s), especially0.001 (τ˜1 s) as a typical value; and for β₆ in the range of 0.1 to0.0001 (τ˜100 ms to 10 s), especially 0.001 (τ˜1 s) as a typical value.

The signal calculated in the just described manner is substantially ameasure for the sum of the change of the measuring signal x and thenoise of the measuring signal x. With a high pass filter 31 and afurther average filter 33, the independent signal SSN is now calculatedfrom (slow) changes of the measuring signal x. This is thus a measurefor the noise of the measuring signal x, and by comparison with thesignal SS, one obtains the desired switching signal SC according to theconditions indicated above.

It has been shown that the damping factors β₄ for example should lie ina range of 0.005 to 0.0005 (τ˜200 ms to 20 s), in particular should beequal to 0.0005 (τ˜2 s).

The output signal of the average filter 33 is connected for scaling to amultiplier unit 34, whose second input is acted on with a scaling factorCS for producing the output signal SSN. It has been shown that thescaling factor CS has a value of 50, for example.

The task of the high pass filter 34 is separating noise and slow changesin the measuring signal x. Assuming that the noise of the measuringsignal is normally distributed over the assessable frequency range of0-1 kHz (with a typical sampling interval ΔT of 1 ms), a high passfilter 34 in accordance with the following configuration has proven tobe suitable:

Filter type: high pass filter

design method: elliptical

sampling frequency: 1 kHz

cutoff frequency in pass band: 400 Hz

oscillations in pass band range: 3 dB

cutoff frequency in stop range: 250 Hz

damping in stop range: 73 dB

Under these conditions, a fourth-order high pass filter is provided,which can be implemented problem-free, can be designed and implementedwith acceptable expense.

FIG. 9 shows a further embodiment of the present invention, in which thefilter unit 10 according to the invention switches optimally quickly andaccordingly abruptly between the two damping factors α₁ and α₂.

It is conceivable that this abrupt switching is not tolerated by all ofthe subsequent controllers in the control unit 5 (FIG. 1). Therefore,the switching between α₁ and α₂ according to the now further embodimentof the invention can be less abrupt by activation of a fade-in/fade-outoption. Toward this end—as is plain from FIG. 9—a transition unit 51 isprovided between the determining unit 50 and the filter unit 10. Thetransition unit 51 is assigned two additional parameters F_(in) andF_(out).

In the following, the function of the transition unit 51 is explained:the two additional parameters F_(in) and F_(out) define two timespans,which are used in switching of the damping factor of α₁ to α₂ or viceversa, wherein depending on the transition direction either the timespanF_(in) or the timespan F_(out) is definitive: If the switch has to bemade from α₂ to α₁ (thus a pressure change occurs), the timespan F_(in)is used during which a soft transition from α₂ to α₁ is carried out. Inthe reverse direction thus when stable pressure conditions againdominate after a pressure change the switch must be from α₁ to α₂. Thishappens according to this embodiment likewise no longer abruptly, butwithin the timespan defined by F_(out). Again a “softer” transition fromα₁ to α₂ occurs.

It has been shown that for the two timespans F_(in) or F_(out), forexample, the following values are suitable:

For the timespan F_(in) in the range of 0 to 100 ms, especially 10 ms asa typical value; and for the timespan F_(out) in the range of 0 to 10 s,especially 1 s is a typical value.

1. A method of determining a pressure a pressure cell (2), wherein the method consists in the fact, that a measuring signal (x) is determined that is at least proportional to a measured pressure in the pressure cell (2), that an output signal (y) is produced using a filter unit (10) that has a transfer function from the measuring signal (x), in that a noise signal contained in the measuring signal (x) is at least reduced and preferably eliminated, and is characterized in that a time change of the measuring signal (x) is determined and the transfer function is set in the function of the time change of the measuring signal (x).
 2. The method according to claim 1, characterized in that the pressure in the pressure cell (2) is set at least proportional to the output signal (6).
 3. The method according to claim 1, characterized in that the transfer function at least in a first order has a low pass characteristic, wherein its time constant is set in the function of the time change (Δx/ΔT) of the measuring signal (x).
 4. The method according to claim 3, characterized in that an average value of the measuring signal (x) is determined, that a difference signal (3) is determined by a difference formation between the measuring signal (x) and the average value of the measuring signal (x), and that the time change (Δx/ΔT) of the measuring signal (x) is derived at least from the difference signal e.
 5. The method according to claim 4, characterized in that the average value of the measuring signal (x) is determined using an exponential average filter, which is defined for a time-discrete measuring signal (x) by f _(n)=β₁ ·x _(n)+(1−β₁)·f _(n−1) wherein f_(n) is the time-discrete average of the measuring signal (s), β₁ is a variable, x_(n) is the time-discrete measuring signal (x), and n is a time-dependent index, wherein the variable β₁ especially has a value between 1 and 0, especially preferably between 1 and 0.1, and most especially between 0.85 and 0.95.
 6. The method according to claim 4, characterized in that the time change (Δx/ΔT) of the measuring signal (x) is determined by formation of an average of the difference signal (e).
 7. The method according to claim 4, characterized in that the time change (Δx/ΔT) of the measuring signal (x) is determined using an exponential average filter, which is defined for a time-discrete difference signal (e) by (Δx/ΔT)_(n)=β₂ e _(n)+(1−β₂)·(Δx/ΔT)_(n−1) where (Δx/ΔT) is the time-discrete time change of the measuring signal (x), β₂ is a variable, e is the time-discrete difference signal and n is a time-discrete index, wherein the variable β₂ especially has a value between 1 and 0, especially preferably between 0.5 and 0.01, and most especially preferably between 0.05 and 0.15.
 8. The method according to claim 1 , characterized in that the transfer function is defined by the formula y _(n) =α·x _(n)+(1−α)·y _(n−1) wherein y is the time-discrete output signal, x is the time-discrete measuring signal, α is a variable whose value depends on the time change of the measuring signal (x), and n is a time-dependent index.
 9. The method according to claim 8, characterized in that the measuring signal (x) is processed in a fast path (20) for producing an output pulse (FC), wherein the output pulse (FC) of the fast path (20) is active at least as long as measuring signal change measured during at most three sampling intervals is greater than the noise measured in the same time period in the measuring signal (x) or in the measuring signal change.
 10. The method according to claim 9, characterized in that the measuring signal (x) further is processed in a slow path (30) for producing a switching signal (SC), wherein the switching signal (SC) of the slow path (30) is active at least as long as change in the measuring signal (s) measured for longer than 2*TS is greater than the noise in the measuring signal (x) measured in the same time period or in the measuring signal change, wherein TS is a predetermined minimal pulse width of the output pulse (FC) and that the variable α obtains a value depending on an OR operation between the output pulse (FC) and the switching signal (SC).
 11. The method according characterized in that the variable α at least after a predetermined transition time after a switching process assumes either the value α₁ or the value α₂, wherein the value for α₁ lies especially in the range of 0.01 to 0.9 and wherein the value for α₂ lies especially in the range of 0.0001 to 0.01.
 12. The method according to claim 8, characterized in that switching from a value α₁ to a value α₂ takes place over a timespan F_(in) and/or that switching from a value α₂ to a value α₁ takes place over a timespan F_(out).
 13. A measuring signal arrangement with a pressure cell (2) and a membrane pressure measuring cell (2) that is functionally connected to the pressure cell (2) and produces a pressure-dependent measuring signal (x), which is applied to a filter unit (10) having a transfer function for producing an output signal (y), characterized in that a time change of the measuring signal (x) may be determined and that the transfer function may be set in the function of the time change of the measuring signal (x).
 14. The measuring cell arrangement according to claim 12, characterized in that the output signal (y) may be used for setting the pressure in the pressure cell (2).
 15. The measuring cell arrangement according to claim 13, characterized in that the transfer function has at least in the first order a low pass characteristic, wherein its time constant may be set in the function of the time change (Δx/ΔT) of the measuring signal (x).
 16. The measuring cell arrangement according to claim 15, characterized in that an average of the measuring signal (x) may be determined, that a difference signal (e) may be determined by means of a difference formation between the measuring signal (x) and the average of the measuring signal (x), and that the time change (Δx/ΔT) of the measuring signal (x) may at least be derived from the difference signal (e).
 17. The measuring cell arrangement according to claim 16, characterized in that the average of the measuring signal (x) may be determined using an exponential average filter, which is defined for a time-discrete measuring signal (x) by f _(n)=β₁ ·x _(n)+(1−β₁)·f _(n−1) wherein f_(n) is the time-discrete average of the measuring signal (x), β₁ is a variable, x_(n) is the time-discrete measuring signal (x), and n is a time-dependent index, wherein the variable β₁ especially has a value between 1 and 0, especially preferably between 1 and 0.1, and most especially preferably between 0.85 and 0.95.
 18. The measuring cell arrangement according to claim 17, characterized in that the time change (Δx/ΔT) of the measuring signal (x) may be determined by formation of an average of the difference signal (e).
 19. The measuring signal arrangement according to claim 16, characterized in that the time change (Δx/ΔT) of the measuring signal (x) may be determined using an exponential average filter, which is defined for a time-discrete difference signal (e) by (Δx/ΔT)_(n)=β₂ e _(n)+(1−β₂)·(Δx/ΔT)_(n−1) wherein (Δx/ΔT) is the time-discrete time change of the measuring signal (x), β₂ is a variable, e_(n) is the time-discrete difference signal (e) and n is a time-dependent index, wherein the variable β₂ especially has a value between 1 and 0, especially preferably between 0.5 and 0.01, and most especially preferably between 0.05 and 0.15.
 20. The measuring cell arrangement according to claim 14, characterized in that the transfer function is defined by the formula y _(n) =α·x _(n)+(1−α)·y _(n−1) wherein 6 is the time-discrete output signal, x_(n) is the time-discrete measuring signal (x), α is a variable whose value depends on the time change of the measuring signal (x), and n is a time-dependent index.
 21. The measuring cell arrangement according to claim 20, characterized in that that the measuring signal (x) is applied to a fast path (20) to produce an output pulse (FC), wherein output pulse (FC) of the fast path (20) is active for at least as long as the measuring signal change measured during at most 3 sampling intervals is greater than the noise measured in the measuring signal (x) or in the measuring signal change in the same time period.
 22. The measuring cell arrangement according to claim 21, characterized in that the measuring signal (x) is further applied to a long path (30) for producing a switching signal (SC) wherein the switching signal (SC) of the slow path (30) is active at least as long as the change in the measuring signal (x) measured for longer than 2*TS is greater than the noise measured in the same time period in the measuring signal (x) or in the measuring signal change, wherein TS is a predetermined minimal pulse width of the output pulse (FC) and that the variable α obtains a value depending on an OR operation between the output pulse (FC) and the switching signal (SC).
 23. The measuring cell arrangement according to claim 20, characterized in that the variable α at least after a predetermined transition time following a switching process assumes either the value α₁ or the value α₂, wherein the value for α₁ especially lies in the region of 0.01 to 0.9 and wherein the value for α₂ especially in the region of 0.0001 to 0.01.
 24. The measuring cell arrangement according to claim 20, characterized in that between the filter unit (10) and the decision unit (12) a transition unit (51) is provided, in which switching from a value α₁ to a value α₂ over a timespan F_(in) takes place and/or switching from a value α₂ to a value α₁ over a timespan F_(out). 